Heme Biosynthesis in Escherichia coli Heme and its related cyclic tetrapyrroles are the prosthetic groups of the hemoglobins, cytochromes, peroxidases, chlorophylls and Vitamin B12. Many of the steps in the biosynthetic pathway of the cyclic tetrapyrroles are highly conserved. The pathway, by necessity is under light metabolic control. Some of the mechanisms of control in various organisms are under active investigation. In the process of cloning and sequencing two genes of the heme biosynthetic pathway hemA and hemB, we demonstrated a unique mechanism of control of the activity of porphobilinogen deaminase (PBG-D), a key enzyme in the pathway, namely that the expression of hemB is required for the activity of PBG-D. In addition, we are actively investigating the control of expression of the genes involved in the synthesis of the first committed intermediate of the pathway, 5- aminolevulinic acid (ALA). We elucidated the pathway of the biosynthesis of ALA in E. coli and showed by both biochemical and genetic techniques that ALA is made by the C5 pathway and not by the C4 as previously believed. We have also identified and characterized a hemE mutant and are in the process of cloning and sequencing it. We have over-expressed hemB, the structural gene for ALA dehydratase and characterized this enzyme. It is proposed to clone the rest of the uncloned genes of heme biosynthesis in E. coli and to study by in vivo and in vitro techniques, the control (transcriptional, translational and enzymatic) of the heme biosynthesis pathway.